1. Field of the Invention
The present invention relates to the field of solar cells, solar power systems and methods.
2. Prior Art
Solar Cell Efficiency Loss Mechanisms
Today's solar cells operate substantially below the theoretical efficiency level established by Sockley-Queisser Model (SQ-Model [W. Shockley and H. J. Queisser, “Detailed Balance Limit of Efficiency of p-n Junction Solar Cell”, J. App. Phys., Vol. 32, pp. 510-519, March 1961]). The solar cell designs described herein can exceed the limit established by the SQ-Model. In order to improve the solar cell efficiency toward exceeding the SQ-Model, it is important to understand the mechanisms that cause the degradation in solar cell efficiency. FIG. 1 (adapted from “Third Generation Photovoltaics: Advanced Solar Energy Conversion”, M. A. Green, Springer, N.Y., 2003, pp. 35-43) illustrates these efficiency degradation mechanisms in the single junction solar cell. Referring to FIG. 1, the efficiency loss mechanisms in a solar cell include the effects listed below.
{circle around (1)} Represents incoming photons with energies (Ep) below the band-gap (labeled Eg) of the device that are not absorbed, thus their energy is not converted into current by the solar cell.
{circle around (2)} Represents incoming photons with energies above the band-gap which are absorbed but lose their excess energy as heat due to the relaxation of the photo-excited electrons and holes (carriers) to the conduction band minimum (CBM) and the valance band maximum (VBM); respectively, by producing phonons (represented in FIG. 1 by the dashed lines). In this loss mechanism, first the photo-excited carriers having energies above the solar cell material band-gap will equilibrate with other carriers to form a carrier population that can be described by a Boltzman distribution (see FIG. 2). At this point the temperature defining the carrier distribution would be above the material lattice temperature and hence the carriers are referred to as “hot carriers”. Typically the additional energy associated with the elevated temperature is contained mainly by the electron due to its lower effective mass. In a typical solar cell, the hot electrons will equilibrate with the cell material lattice by giving off their excess energies to the cell material lattice by producing phonons during their cooling time τc period (see FIG. 2). These phonons then interact with other phonons and the absorbed photon energies Ep in excess of the cell material band-gap Eg are lost to heat and hence are not converted into voltage by the solar cell. Depending on the carrier mobility and crystal lattice characteristics of the cell material, the carrier cooling time τc occurs in a timescale of a few picoseconds to a few nanoseconds (see FIG. 2). As illustrated in FIG. 2, by the end of the carrier cooling time τc the photo-excited carrier distribution will coalesce to a narrow energy distribution of electrons and holes near the edges of the conduction and valence bands of the cell material; CBM and VBM, respectively. This final stage of the photo-excited carrier lifetime would typically last for a few microseconds (carrier recombination time τr) as the photo-excited carriers systematically recombine giving their residual gained energy to photons. In order for a conventional solar cell to be able to convert the energy of the photo-excited carriers to electric energy, the photo-excited carriers must be separated and transported toward the cell contacts before they recombine, meaning before the elapse of the carrier recombination time τr. The design parameters of conventional solar cells are typically selected to achieve the carrier transport characteristics needed to transport the photo-excited carriers to the cell contacts before they recombine; i.e., before the elapse of the carrier recombination time τr. From the above discussion, the solar photon energy given to the photo-excited carriers would be dissipated in two main stages; namely carrier cooling and carrier recombination. During the former of these two main stages; namely the cooling stage, the photo-excited carriers give their energy in excess of the material band-gap energy separation to phonons while during the latter stage; namely the recombination stage, the photo-excited carriers give their residual energy, which is typically equal to the material band-gap energy separation, to photons through radiative recombination.
{circle around (3)}Represents photo-excited carriers (electrons and holes) which recombine radiatively before being extracted and produce either a photon with energy equal to the band-gap or possible multiple photons with energies less than the band-gap. This radiated energy is not necessarily lost as these photons can be reabsorbed. However, these radiated photons, unless confined, will be re-emitted from the cell back toward the incoming sunlight and lost forever—an effect that ultimately restricts the maximum efficiency that can be achieved by the solar cell. In most bulk semiconductor materials, the timescale of carrier recombination is typically less than a few microseconds (see FIG. 2). For the solar cell to be efficient, most photo-excited carriers must be transported to the cell contacts and extracted before the carriers recombine, although at that point the energy separation of the electrons and holes to be extracted from the cell would only be comparable to the cell material band-gap energy.
{circle around (4)} Represents photo-excited carriers (electrons and holes) which recombine non-radiatively with the help of electronic states within the band-gap. These states are typically caused by defects in the solar cell material lattice structure or by impurity atoms, and the resultant non-radiative carrier recombination would produce phonons, thus the energy of the absorbed solar photons that caused the excitation of these carriers is transferred to heat rather than being converted into current by the solar cell. This loss mechanism is one of the main efficiency loss mechanisms in monolithic multi-junction stack solar cells where the lattice mismatch between successive layers can create lattice misfit dislocations which can severely diminish the solar cell performance by creating additional regions at the stacked cells boundaries where carriers can non-radiatively recombine.
{circle around (5)} Represents photo-excited carriers (electrons and holes) which are not effectively extracted by the solar cell contacts. This loss mechanism is typically caused by high resistance at the cell contacts that tends to cause inefficiency in extracting the carriers out of the cell, thus ultimately limiting the maximum efficiency that can be achieved by the solar cell. This mechanism is also an important efficiency loss mechanism in monolithic multi-junction stack solar cells as there are only two contacts to extract the current from the multi-junction stack, making the lowest individual current producing cell structure within the stack limit the total current of the entire multi-junction stack. Also this loss mechanism is the main culprit behind the difficulty in extracting hot carriers from solar cells as these carriers tend to rapidly cool down at the contact, an effect that causes the hot carriers to congregate near the cell junction, making it difficult to extract these carriers before they cool down.
In addition to the above efficiency loss mechanisms, the theoretical model typically used to predict solar cell efficiency, namely the SQ-Model, includes certain assumptions that limit the perceived efficiency that can be achieved by solar cells—thus somewhat preventing solar cell designers from pushing their designs to their true limits. The most relevant of these assumptions are listed below.
1. The input is the un-concentrated solar spectrum;
2. Each incident solar photon will produce only one electron-hole pair;
3. The cell can achieve only one Quasi-Fermi Level (QFL) separation;
4. The cell is operating at thermal equilibrium with the cell and carrier temperatures being equal; and
5. The cell is operating in steady state current flow condition.
The solar cell efficiency limit based on the SQ-Model is calculated by examining the amount of electrical energy that can be extracted per incident solar photon. Since the incident solar photon excites an electron from the solar cell material valence band to its conduction band, only photons with more energy than the cell material band-gap will produce power. That means that the theoretical conversion efficiency of a silicon (Si) solar cell with band-gap at 1.1 eV would be less than 50% since almost half of the photons within the solar spectrum have energy below 1.1 eV. Considering the difference in the energy between the solar photon being absorbed from the sunlight at 6000° K and the cell operating at 300° K, the SQ-Model equilibrium assumption would imply that any solar photon energy above and beyond the cell material band-gap energy would be lost. Since blue photons have roughly half of the solar energy above 1.1 eV, the combination of these two assumptions would result in a theoretical efficiency peak performance of approximately 30% for a single junction Si solar cell.
In addition to the efficiency limitations implied by the SQ-Model assumptions, there are several other considerations that are implied by the material system used in the solar cell, such as the carrier production rate and mobility characteristics of the material system. These types of considerations do not affect the efficiency of the cell under normal conditions, but introduce further limits under certain conditions (for example, an increase in the number of incident solar photons due to concentration). The first of these two effects, namely, the carrier production rate, sets a saturation (or a maximum) level on the rate in which carriers are produced within the cell material as a result of photo-excitation, and hence limits the amount of energy that can be extracted from the cell. Intuitively, as the number of solar photons incident on the cell surface increases, the amount of energy that can be produced by the cell should increase. However, such is not the case in some material systems (such as Si, for example) in which, due to low electron mobility, the number of holes increases with the increase of photo-excitation at a rate that is much faster than electrons. This hole and electron density imbalance will cause photo-excited electrons to recombine with the abundantly available holes before they can be extracted, thus placing a limit on the number of electron/holes that can be extracted from the cell. In Si cells, this limiting rate (equilibrium) is reached at less than 2-sun of incident light. As a result, when twice as much sunlight is incident on the surface of a Si solar cell, the carrier production rate would only be slightly higher than with 1-sun, making the ratio of the input energy to output energy lower, which represents a much lower efficiency. For that reason Si solar cells are not effective with solar concentrators.
The electron mobility in other material systems, such as gallium arsenide (GaAs) or gallium nitride (GaN), is much higher than that in silicon, enabling photo-excited electrons to reach the cell junction more quickly, thus alleviating the occurrence of holes/electron density imbalance and reducing the chances that electrons and holes will recombine before they can be extracted, which in turn would allow an increase in the number of incident solar photons to continue to result in an increase in the number of photo-excited carriers before equilibrium is reached. This increase in electron mobility, therefore, would allow solar cells made from such material systems to have an increased efficiency under concentrated sunlight.
The discussion in the following sections of this disclosure is intended to highlight several novel design approaches that would circumvent many of the efficiency loss mechanisms explained above and therefore allow the alternating bias solar cell designs described in the following sections to offer extremely high solar power conversion efficiency. Subsequent sections of the disclosure will discuss the cost/efficiency performance of multiple embodiments of the alternating bias solar cells and compare it with the performance achieved by current conventional solar cells. The objective of the discussion below is therefore to show that the cost/efficiency performance predicted to be achieved by the alternating bias solar cell of this invention could offer a solar energy cost per kWh that reaches the 3rd Generation (3G) target of the photovoltaic solar cell industry.
Harnessing Hot Carriers
As explained earlier, one of the primary loss mechanisms in solar cells is the loss of incident solar photons with energy above the cell material band-gap due to hot carrier relaxation, loss mechanism {circle around (2)} in FIG. 1. Although it is theoretically possible for hot electrons to be separated and collected at contacts before cooling occurs, this is not observed in conventional solar cells due to the fast thermalization of hot carriers (short cooling time, τc). Currently there are two concepts being envisioned by researchers in the field for increasing solar cell efficiency utilizing hot carriers, namely, direct extraction using selective energy contact (SEC) and multiple exciton generation (MEG). Both concepts rely on first slowing down the carrier cooling, but the hot carrier energy is harnessed in different ways.
Theoretical treatments of direct hot carrier extraction using SEC, which is illustrated in FIG. 3A (“Solar Energy Material and Solar Cells”, P. Würfel, 46 (1997), pp. 43-52), is widely published and have shown that substantial solar cell efficiency increase near the thermodynamic limit of 68% from a single junction cell (“The Physics of Solar Cells” J. Nelson, Imperial College Press, 2003, pp. 309-316) would be possible if hot carriers could be effectively extracted. However, it is not easy to separate hot (high energy) electrons and holes (carriers) to the cell contacts because these hot carriers tend to lose their high energy through the interaction with phonons that cause the high energy of hot carriers to be rapidly lost as heat. The entire concept behind maintaining the photo-excited hot carrier population within the cell in both SEC and MEG approaches is to minimize the electron-phonon interactions. However, in the vicinity of metal contacts, it is very easy for the hot carriers to cool down through the large number of available electronic states in the contacts. Therefore, hot carriers would typically tend to congregate near the cell junction, making it even more difficult to transport and extract these carriers before they cool down. In typical solar cell materials, the distance the hot carriers can travel through the cell material before cooling tends to be very short (less than a micron), making it more difficult to transport the hot carrier to the cell contacts before they cool down.
It should be noted that the principle of the SEC approach is to use a contact material having a narrow density of states with large band-gap between the next available states (“Solar Energy Material and Solar Cells”, P. Würfel, 46 (1997), pp. 43-52). However, a narrow density of states would also yield extremely low electron mobility and therefore there must be some level of compromise between the narrowness of the density of states and maintaining high enough conductivity through the contact. An additional issue that will need to be addressed before SEC becomes feasible is the geometry of the cell and its associated contacts. Given that the distance the hot carriers can travel before cooling is typically very short, it would be necessary to design the cell structure such that carriers are generated very close to the SEC contact to ensure the carriers do not cool before being collected at the contact. Therefore, very short absorber regions and/or convoluted surfaces may be required to minimize the distance the hot carriers will have to travel (“Third Generation Photovoltaics: Advanced Solar Energy Conversion”, M. A. Green, Springer, N.Y., 2003, pp. 35-43).
The other possibility for increasing the efficiency of solar cells utilizing hot carriers is through MEG (“Third Generation Photovoltaics Advanced Solar Energy Conversion”, M. A. Green, Springer 2006, pp 81-88). In this case, the excess energy of the hot electrons is used to create additional excitons, i.e., bound electron-hole pairs. The hot electron must have the energy of at least two times the band-gap Eg to create one additional electron-hole pair. This process is not limited to electrons with energy of twice the band-gap, but it can also be extended to electrons with higher energies. Under 1-sun AM1.5 spectrum, the predicted theoretical efficiency of MEG-enhanced cells is over 44%, while under maximum sunlight concentration, the efficiency can approach that of SEC cells. Although MEG can occur in bulk semiconductors, its probability of occurrence is so low that it does not contribute much to the efficiency of the cell (“Third Generation Photovoltaics”, Gregory F. Brown and Junqiao Wu, Laser & Photon Rev., 1-12 (2009), published online: 2 Feb. 2009).
As stated earlier, slowing the cooling of hot carriers is prerequisite for both the SEC and MEG approaches and the most widely pursued way for achieving this by ongoing research in the field is through the use of quantum confinement structures. There are cases wherein hot carriers' cooling time exceed the typical cooling time in bulk semiconductors. This phenomenon is expected to occur in many material systems incorporating quantum confinement structures. First, multiple quantum wells (MQWs) and quantum dots (QDs) were studied and found to have hot carrier cooling times much larger than that of bulk semiconductors (“Third Generation Photovoltaics”, Gregory F. Brown and Junqiao Wu, Laser & Photon Rev., 1-12 (2009), published online: 2 Feb. 2009). Hot carrier cooling times approaching a few tens of nanoseconds have been observed in these types of structures. This increase has been attributed to the phenomenon known as the phonon bottleneck effect in quantum structures. Typically hot electrons cool through interactions with optical phonons and due to the presence of quantum confinement, a non-equilibrium level of optical phonons can be created. Due to the phonon bottleneck effect caused by the quantum confinement aspects of MQWs or QDs, these optical phonons cannot equilibrate with the lattice fast enough, thereby slowing the further cooling of hot electrons—extending their cooling time τc (see FIG. 2). Within the two dimensional quantum confinement of MQWs, the phonon bottleneck effect occurs at high carrier photo-excitation densities that require relatively high illumination levels, such as those typically occurring under sunlight concentration. However, due to the three dimensional quantum confinement aspects of QDs, the phonon bottleneck effect is expected to occur under all illumination levels. As a result of the slowed down cooling of hot carriers in MQWs and QDs, these types of device structures are expected to play an important role in hot carrier extraction.
In principle, in the SEC hot carrier cell illustrated in FIG. 3A, with the hot carrier cooling being slowed down by the quantum confinement structures incorporated with the cell material, there would be sufficient time to transport the carriers to the contacts while they are still hot where they can be collected at their high energy level by the narrow density of states of the contacts. This would theoretically allow an increase in the photovoltage yielded by the cell. However, the increased photovoltage at the cell contacts would tend to counteract the built-in potential Vbi responsible for transporting the carriers to the cell contacts. As a result, the time it would take to transport the carriers to the contacts (carrier extraction time) would be substantially increased to the point that could reach the carrier recombination time τr; meaning the carriers would recombine before reaching the contacts due to the weakening of the cell carrier built-in transport mechanism, which in turn would result in a substantial reduction in the photocurrent yielded by the cell with net energy extraction that is not substantially greater than that of a conventional cell. So although SEC hot carrier cells can in theory generate higher photovoltage, the realized increase would most likely be more than offset by the reduction in photocurrent that is a direct consequence of the prolonged carrier extraction time.
Given the increased attention to renewable energy, in particular photovoltaic (PV) solar cells, there is an increasing demand to increase the efficiency of PV cells without substantially increasing their cost. Hot carrier PV solar cells have been theoretically predicted to be able to offer a substantial increase in the PV cell efficiency, but to date none of these predictions have been realized. The two hot carrier solar cell approaches discussed earlier require means for increasing the carrier cooling time, which require the inclusion of quantum confinement structure within the cell material, which in turn would very likely increase the cell cost. The benefits of MEG hot carrier cells can only be realized under very high solar concentration that renders that approach impractical. In addition to requiring special types of contacts that often require the use of a multi layer superlattice, the SEC hot carrier solar cell approach appears to suffer from a built-in deficiency that counteracts its ability to reach a higher energy efficiency than what a conventional PV cell can offer. Given the high demand for more efficient and less costly PV solar cells and the weaknesses of the approaches currently being pursued to attain this objective, a PV solar cell approach that can effectively realize higher efficiency without significant increase in the solar cell will most likely have a substantial market value.